There are many applications using terahertz (THz) frequency range radiation. For example, THz radiation can be used for characterization of a variety of properties of solid and liquid materials, such as photoconductivity, dispersion, absorption, refractive index, etc. THz radiation can penetrate dry, non-metallic and non-polar objects like plastic, paper, textiles, cardboard, semiconductors and non-polar organic substances, therefore THz radiation can be used for safe package inspection instead of x-rays, e.g., to look inside boxes, cases etc. THz radiation can also be used for industrial process control, food inspection, biology and medicine.
THz radiation may be generated or detected using so-called photoconductive antennas, which comprise two electrodes provided on the surface of a photoconductive substrate. To generate radiation such an antenna can, for example, be excited by directing a light pulse onto such a device. When a bias voltage is applied to the electrodes, a photogenerated current flows between the electrodes, which in turn results in the emission of broadband radiation with frequencies up to the THz range. Alternatively, the pulse laser pump can be replaced with two CW lasers of slightly different frequencies so that when mixed in the active region of the photoconductor they produce a mixing signal also in the THz range.
Materials suitable for the photoconductive substrate are typically semiconductor materials which are grown at low temperatures (typically 200° C.-300° C. rather than the more usual growth temperatures in the region of 600° C.), or materials which have been implanted with ions after growth. Examples of such materials include, but are not limited to, low temperature GaAs (denoted LT GaAs), arsenic implanted GaAs (As—GaAs), LT InGaAs, and LT AlGaAs.
A typical prior art arrangement 10 for generation of THz radiation is shown in FIG. 1. The arrangement includes two strip electrodes 11, 12 mounted on a photoconductive substrate 13 and interconnected with DC bias source 14. The strip electrodes 11, 12 are provided with protrusions 15, 16 facing each other, thereby defining an electrode gap 17 therebetween, which is the active region of the device when illuminated with a light beam 18. As shown in FIG. 1, the protrusions 15, 16 are formed as planar rectangular strip elements with flat edges, however more sophisticated shapes for the strip elements are also known.
For example U.S. Pat. No. 5,729,017 describes pulse generators and detectors for operating at frequencies of the order of 1010 to 1013 Hz (the Terahertz range). These devices rely on electric field interactions with optical beams in biased metal semiconductor microstructures. An electric field is created between metal electrodes on the semiconductor surface and the electric field is enhanced by configuring the electrode gap geometry with sharp electrode features.
THz antennas, in which the electrodes are provided as interdigital arrangement, are known. Referring to FIG. 2, a typical interdigital antenna arrangement 20 is illustrated. The inter-digital antenna arrangement 20 comprises first and second electrodes 21 and 22 with interdigital finger structure mounted on a photoconductive substrate 23. The first electrode 21 comprises a planar main body including a plurality of elongate fingers 24 connected to the electrode's main body 21. The electrode's main body 21 and the elongate fingers 24 form a continuous metallic planar structure. A second electrode 22 is identical to the first electrode 21, but is arranged at 180° with respect to the first electrode 21. The second electrode 22 has corresponding fingers 25. The elongate electrode fingers 24 and 25 are of a width and are spaced apart such that the elongate fingers 24 of the first and electrode 21 can fit within the gaps provided between spaced apart fingers 25 of the second electrode 22. Thus, the contacts are interdigitated due to the interleaving of the electrode fingers.
Since the neighboring fingers 24 and 25 in the antenna arrangement 20 are biased with reciprocal polarity, a direction of a current (shown by a reference numeral 26) flowing in the photoconducting material between two certain neighboring fingers 24 and 25 arranged in the finger array is opposite to the direction of a current (shown by a reference numeral 27) flowing in the photoconducting material between two next neighboring fingers 24 and 25. A problem with these current opposite directions is coherent signal cancellation in the interdigital arrangement 20, and accordingly there is a need for decoupling of the individual fingers 24 and 25 as radiating elements of the arrangement 20, in order to prevent destructive interference of the THz distant fields.
In order to solve the problem of the coherent signal cancellation in an interdigital arrangement, in German patent DE 10 2004 046 123 A1, an interdigital structure was proposed to increase the radiancy of the terahertz radiation emitted by a photoconductive antenna, which has every second finger structure covered by a layer impermeable to the exciting laser light. In this structure, the terahertz waves emitted between the fingers of the interdigital structure have uniform polarisation orientation and constructively overlap in the far field.
German patent No. 10 2006 059 573 describes a THz arrangement in which the optical excitation of the charge carriers in the photoconductive material is limited to every second finger of the inter-digital finger array structure. In order to limit this excitation, the arrangement includes a lens array. The focal points of the individual lenses of the lens array are all located at the surface of the semiconductor material between every second finger of the interdigital finger structure.
International patent application WO2007/112925 describes an antenna array having a plurality of THz antennae. The lateral regions between neighbouring THz antennae are practically free of photoconductive material to prevent occurrence of current between the antennae.